1. Field of the Invention
The invention relates to transparent substrates, more particularly glass substrates, intended to be incorporated in glazing panes and provided with antireflection coatings.
2. Description of the Background
An antireflection coating usually consists of a stack of interferential thin layers, in general an alternation of dielectric-based layers having high and low refractive indices. The function of such a coating, when deposited on a transparent substrate is to decrease its light reflection coefficient and hence to increase its light transmission coefficient. A substrate thus coated therefore has a higher transmitted light/reflected light ratio, which improves the visibility of objects placed behind it. In order to obtain a maximum antireflection effect, it is recommended to provide each of the faces of the substrate with this type of coating.
One of the best-known applications of this type of product is the protection of paintings illuminated by a light placed behind the observer. A glazing pane exhibiting an antireflection effect is also highly advantageous for equipping buildings for example as a shop window, so as to make it easier to distinguish what is in the window when the internal lighting is low compared to the external lighting, or as counter glass.
It will also be useful to employ this type of product as vehicle glazing pane, especially for a car, and most particularly as a windscreen, the standards imposing high levels of light transmission.
What currently limits the use of antireflection glazing panes in buildings or for equipping vehicles is the level of mechanical and chemical durability which is necessary in such applications. This is because the antireflection coating in the glazing pane will lie at least on its face 1, that is to say that face of the glazing pane turned to the outside of the room or of the passenger compartment (conventionally, each of the faces of glass substrates forming a given glazing pane is numbered by starting from the face which is turned towards the outside). Now, this face of the glazing pane is exposed to a great deal of stressing; thus, in buildings, it is subjected to the vagaries of the climate and to cleaning using quite abrasive means and/or using quite corrosive chemicals. The problem of durability is perhaps even more glaring in the case of glazing panes for vehicles--the windscreen is subjected to the abrasive effect of the to-and-fro action of the windscreen wipers and to the various projections of dust particles or gravel particles--and all side windows of vehicles are subjected to repeated rubbing against the rubber lips of the doors.
In fact, hitherto most antireflection coatings proposed are obtained by thin-film deposition using vacuum techniques, for example sputtering-type techniques. This type of deposition technique leads to the formation of thin layers which are of good quality, especially optical quality, but which often exhibit inferior durability compared to that which would be required for the applications envisaged above. Furthermore, these techniques are used in a subsequent operation, that is to say discontinuously on the glass panels once they have been cut up from the ribbon of glass coming from a float production line.